(a) Field of the Invention
The present invention relates to a method and apparatus treating an area of skin using a multipulse laser. More particularly, the present invention relates to a method and apparatus that would enable the CO2 laser beam to be delivered deep into dermis yet cause minimum thermal damage to the epidermal and superficial dermal tissue.
(b) Description of the Related Art
Since it was first invented in 1964, the CO2 laser has been one of the most widely used surgical lasers. The CO2 laser emits an infrared beam at 10,600 nm (nanometers). The beam is invisible to the human eye but has a uniquely high absorption characteristic in water molecules. Since human soft tissue is more than 70% water, the CO2 laser is ideally suited to target the intracellular and extracellular water in the target tissue. When the water in the tissue absorbs laser light at 10,600 nm, the temperature of the tissue almost instantaneously increases until vaporization and ablation occur, surrounded by roughly concentric zones of coagulation, protein denaturation and athermal photobiomodulation, which can be seen clearly in stained histopathological specimens (refer to FIG. 7A).
Referring to FIG. 7B, changes in the staining pattern clearly show the different areas of damage. In a carbonization (Ca) area, tissue has been vaporized. In a coagulation (Co) area, where the tissue is literally cooked, very few cells will survive and most are already dead. In an area of protein denaturation (PD), there is a mixture of dead, damaged, and surviving cells. This zone is very important in starting the wound healing process. In an NT (normal-appearing tissue) area, consists of cells which have been directly stimulated by low photon density of laser energy exist and this layer is even more important for ensuring a good transition between the stages of wound healing.
Because of its efficacy in creating a layer of coagulative necrosis, by which small blood and lymphatic vessels are sealed, the CO2 laser has become an ideal tool for bloodless surgery. In addition to incision and ablation, lowering of the incident power density of the CO2 laser can also give selected nonablative effects.
In the middle of the 1990s, the CO2 laser started to be used for scar revision and wrinkle removal. The water specific absorptive property of the CO2 laser energy enables it to be used to ablate superficial tissue with the deposition of a controlled layer of residual thermal damage (RTD). The advantage of this method is that the tissue can be ablated layer by layer, with the RTD zone providing coagulation of small blood vessels thus giving a dry operative field. By adjusting the amount of energy delivered, the operator can control the depth of ablation to induce a deep or superficial peeling effect.
However, because the CO2 laser ablates the epidermis and sometimes even goes down to the mid-dermis, over-aggressive CO2 laser treatment has been associated with delayed healing time for reepithelialization. Even in cases of appropriate application, severe edema and crusting are seen in the immediately post-peel days with prolonged erythema and sometimes post inflammatory hyperpigmentation during the healing phase. After ablative CO2 laser resurfacing, patients can experience weeks and sometimes months of recuperation time.
To overcome the disadvantages of ablative resurfacing for skin rejuvenation, nonablative resurfacing, known as nonablative skin rejuvenation, was developed. The newer methods use systems which employ shorter wavelengths than the CO2 but that can still thermally damage the superficial and upper reticular dermis, but they do so under an intact epidermis which is aggressively cooled with, for example, a cryogen spray or contact cooling with a cooled sapphire lens. This technique enables selective cooling of the epidermis so that when the laser is fired onto the skin, and as the tissue absorbs the laser energy, the epidermal temperature is maintained at levels below the thermal damage threshold. So in effect, the dermis is heated and damaged but the epidermis is preserved. When the dermal wound is undergoing the wound healing process, the natural restorative effect of the intact epidermis can help to promote collagen regeneration and result in the desired rejuvenating effect. The essence of nonablative skin rejuvenation is controlled delivery of damage to the target dermis under an intact epidermis, so that none of the disadvantages of the ablative approach are experienced, and patient recuperation time is virtually nonexistent.
The limitation of current nonablative skin rejuvenation systems is that the laser must utilize some kind of cooling system. Furthermore, the main traditional wavelength for resurfacing, namely 10,600 nm of the CO2 laser, can not be used due to its water absorption characteristics which limit the depth of penetration.
The contact cooling method cannot be effectively used for the CO2 laser due to the presence of water condensation on the contact surface which would absorb the laser energy, and glass or quartz are actually opaque to that wavelength. In the cryogen spray cooling method, the cooling protects the epidermis, but the CO2 wavelength still does not allow sufficient penetration depth. The shallow penetration depth would limit the thermal damage required to be delivered to the deeper dermal regions and thus limit the collagen regenerative effect brought about by the wound healing process in tissue, which is well-recognized as being absolutely necessary for skin rejuvenation. As the CO2 laser is an accepted modality as an ablative resurfacing tool, there is a need to create a system that enables the delivery of the CO2 laser beam deep into the dermis yet produces non-ablative resurfacing clinical effects.